The present invention relates to the general field of nozzles fitted to airplane turbojets. It relates more particularly to separate-stream nozzles fitted with stream mixers that serve to reduce the noise emitted by the jet leaving the nozzle, while also improving the performance of the engine.
More precisely, the separate-stream nozzles of turbojets are generally constituted by a confluent flow nacelle containing a primary cover defining a first annular channel for the flow of a primary stream (or hot stream) coming from the combustion chamber, and a secondary cover disposed concentrically around the primary cover so as to define a second annular channel for the flow of a secondary stream (or cold stream) coming from the fan.
In order to reduce the noise of the jet generated at the outlet from such a nozzle, it is known to use mixers having a special design for encouraging mixing between the primary and secondary streams coming from the turbojet. One of the designs in widespread use at present in civilian turbojets comprises the lobe mixer. The lobe mixer serves to obtain radial shear between the primary and secondary streams, thereby encouraging mixing between those streams. Embodiments of lobe mixers for separate-stream nozzles are described in particular in the following documents: EP 1 141 534; U.S. Pat. No. 5,755,092; and U.S. Pat. No. 5,924,632.
Nevertheless, although a mixer of that type serves to improve the efficiency and the sound performance of turbojets having separate-stream nozzles, it inevitably increases the weight of the nozzle, thereby impacting the dynamics of the engine as a whole and of its connection with the wiring pylon. A lobe mixer is traditionally made of a metal, typically of Inconel® 625. Consequently, it represents a non-negligible extra weight cantilevered out from the engine, leading to an increase in the mechanical load on the flange of the nozzle exhaust casing, in particular in the event of extreme or limiting loads as occur with the loss of a blade or in response to unbalance. This additional weight also generates an increase in the forces at the engine-to-pylon interfaces.